PIC18F258 MICROCHIP [Microchip Technology], PIC18F258 Datasheet - Page 27

• Manufacturer Part Number: PIC18F258
• Description: High Performance, 28/40-Pin Enhanced FLASH Microcontrollers with CAN
• Manufacturer: MICROCHIP [Microchip Technology]

3.0
The PIC18FXX8 differentiates between various kinds
of RESET:
a)
b)
c)
d)
e)
f)
g)
h)
Most registers are unaffected by a RESET. Their status
is unknown on POR and unchanged by all other
RESETS. The other registers are forced to a “RESET”
FIGURE 3-1:
 2002 Microchip Technology Inc.
MCLR
OSC1
V
Power-on Reset (POR)
MCLR Reset during normal operation
MCLR Reset during SLEEP
Watchdog Timer (WDT) Reset during normal
operation
Programmable Brown-out Reset (PBOR)
RESET Instruction
Stack Full Reset
Stack Underflow Reset
Note 1: This is a separate oscillator from the RC oscillator of the CLKI pin.
DD
Instruction
RESET
RESET
Pointer
Stack
RC OSC
2: See Table 3-1 for time-out situations.
On-chip
OST/PWRT
Brown-out
V
Module
Detect
DD
WDT
Reset
(1)
Rise
Stack Full/Underflow Reset
SIMPLIFIED BLOCK DIAGRAM OF ON-CHIP RESET CIRCUIT
OST
PWRT
Time-out
Reset
WDT
10-bit Ripple Counter
External Reset
10-bit Ripple Counter
BOREN
Power-on Reset
SLEEP
Preliminary
state on Power-on Reset, MCLR, WDT Reset, Brown-
out Reset, MCLR Reset during SLEEP and by the
RESET instruction.
Most registers are not affected by a WDT wake-up,
since this is viewed as the resumption of normal oper-
ation. Status bits from the RCON register, RI, TO, PD,
POR and BOR are set or cleared differently in different
RESET situations, as indicated in Table 3-2. These bits
are used in software to determine the nature of the
RESET. See Table 3-3 for a full description of the
RESET states of all registers.
A simplified block diagram of the on-chip RESET circuit
is shown in Figure 3-1.
The Enhanced MCU devices have a MCLR noise filter
in the MCLR Reset path. The filter will detect and
ignore small pulses.
A WDT Reset does not drive MCLR pin low.
PIC18FXX8
S
R
Enable PWRT
Enable OST
DS41159B-page 25
Q
Chip_Reset
(2)